Electric motor driven vehicles, in particular golf cars, have many unique performance requirements that pose difficult obstacles to the design of control systems for direct current motors used as the primary means of propulsion. With the advent of electronic controls these obstacles can be confronted and through the use of the system of this invention they can be eliminated.
Drive systems of the prior art have generally employed motors configured with their armature and field coils connected in series because this type of motor provides beneficial starting characteristics. More recently, shunt type motors, called such because they have their armature and field windings connected in parallel to a common source of voltage, are replacing the series wound motors because shunt motors offer a greater flexibility in controlling motor performance. By adapting the input power to the motor to provide independent excitation of the armature and field windings further flexibility can be achieved. It is the purpose of this invention to control an independently excited shunt motor to provide enhanced performance in the propulsion of a golf car.
Shunt motors tend to have a characteristic response curve, namely a specific speed for a specific load. This is a result of a balancing of the input voltage and the back emf generated by the motion of the motor armature through the magnetic field established by the energized field coil. Speed and torque can be conveniently controlled by the adjustment of the voltage or current across or in either the armature or the field winding or both. Although motor control can be obtained through either the armature current or the field current, it is advantageous to rely on the variation of field current because of the lower current levels. The use of field current enables the use of low power, less expensive components that increase efficiency. It is the purpose of this invention to provide a system of control which allows operation of the motor along its full range of response characteristics.
Basic speed control for such motors involves the use of a chopper circuit to supply pulses of voltage to the armature which are varied in duty cycle from 0 to 100% according to the desired speed in response to the manually operated throttle. Further refinement of the control is provided by an H bridge circuit operatively connected to vary the current in the field winding. These circuits are well known in the art.
The more sophisticated motor control circuits are now based on the operation of discrete logic or microprocessors and can function to provide a variable torque at near Constant speed. An example of one such system particularly adapted to the control of electric vehicles driven by a separately excited shunt wound motor is described in U.S. Pat. No. 5,264,763. In the system of the '763 patent, the armature winding is controlled by modulating the pulse width of a chopper exciter circuit 17 while the field current is controlled by the H bridge circuit 16. The system of the '763 patent adjusts the armature and field voltage in response to the speed of the driven wheel. It is an object of this invention to realize the full potential of this type of control circuit to enhance the performance of the drive system for a golf car or other vehicle through the adjustment of the field current.
A great deal of design effort has been exerted to utilize the power generation characteristic of an electric motor to provide a braking action in a motor used for driving a vehicle. This can be accomplished in many ways for example by reversing the voltage on the field or armature winding or by providing a return path for current through the armature upon disconnecting the input power to the armature. It is a purpose of this invention to use this characteristic to enhance the performance of a golf car in its normal use.
The operation of a golf car is unique because of the uneven terrain which is continuously traversed and the continuous need for stopping and parking the car. Although a very simple control circuit can provide constant speed propulsion along a flat path, most courses are designed to challenge the golfer. Hills, valleys, bunkers, ditches and the like, all of which need to be navigated by the golf car, are a standard part of any course.
In using electric motor powered golf cars, there is a continuous need for the driver to be alert to an overspeed condition on down hill runs and to apply the mechanical braking system to slow down the vehicle. In gas powered golf cars the compression of the engine cylinders provides some restraining force against overspeed when the car is proceeding downhill. The electric motor has no similar restraint unless the regenerative nature of the armature can be actuated to provide auxiliary braking. It is a purpose of this invention to provide an efficient mechanism for automatically utilizing regenerative braking during an overspeed condition to provide auxiliary braking.
As in any vehicle many functions of a golf car depend on the actions of the driver. When a golf car is stopped it should be stopped on level ground and the mechanical brake must be reliably set. If the driver is negligent the car could continue to roll even though the motor is turned off. The irresponsible or forgetful driver could use a mechanism which would sense movement after the car is brought to rest. It is a purpose of this invention to provide a control system having an additional operation alert function which will sense the movement of a car after it comes to rest and initiate regenerative braking in the proper direction to limit the speed of the vehicle.
The driver of a golf car also faces the opposite of the overspeed event when operating the vehicle on an upward incline. In this situation the car will tend to slow down as the load on the motor increases as it proceeds up the grade and the driver will be at the mercy of the characteristic torque versus speed curve of the motor. It is a purpose of this invention to provide a control circuit which can adjust the operational performance of the motor to obtain a full range of available torque while maintaining a near constant speed.